Manufacture of dielectric shapes



Sept. 10, 1946.y G, sMoLAK MANUFACTURE oF DIELECTRIC sHAPEs Filed Dec. 16, 1943 NVE-*JCR fia/ass iva/wr.

Patented Sept. 10, 19.46

f UNITED lsTA'rEsi PATENToFFlcE ,y l ALlwrllxuIJFACTUREi),)17;:`(i).E*o'rmc sHAPEs Y V`George Smolak, Somerville, N. J., assigner tok Johns-Manville Corporation, New York, N. Y., a'corporation ofNew York `Application December 16, 1943,Y Serial No. 514,446

Thisginvention relates to the manufacture of tough and strong structural dielectric sheets or shapes having properties particularly adapted for Y use as switchboard panels and the like.

.Serious diliicultie's have been encountered lby those attempting to solve the problem of providing dielectric panels Yof suitable large size and dielectric properties and having the impact and flexural strength to withstand the severe mechanical strains which are imposed particularly by --The present invention provides a tough and,y

hard surfaced moldedV shape which is highly resistant to moisture penetration, has'goodj dielectric-strength and dielectric stability in moist atmospheres, vpossesses dimensional stability and physical strength throughout a wide temperature range and which resistsY charring and burning on exposureto arcng or short circuiting. t

2 Claims. ,(Cl, 26o-38) moistureabsorption of a panel with changes in proportions of ber and resin; and f Fig. 6 is a perspective View (partly in section) showing a fragment of a dielectric panel manufactured in accordance with the invention.

The first step in preparing structural dielectric sheets in accordance with the present invention is to subject a supply of crocidolite asbestos fibers to a thorough willowing treatment, to open the v fibers, to flberize coarse ber pencils, and to produce Well-'opened fine fibers of average lengthy approximating 1A inch; Afterthe crocidolite lbers have been thoroughly,wil1owed',l they are subjected to a further opening and flulllng'opera-V tion, asfbybeing passed through an lair b1ower,'to' break up any clots ofmatted fibers. Thoroughly opened andfluled fibers are then introduced into a batch mixervand mixed therein with a measured proportion of finely pulverized and screened heat hardenabl'e" B 'stage phenolic 'or` Bakelite resins (for example, type'BRf7095);v It'should be undern stood'that the 'composition of such resins maybe The invention consists in the improved Istruc- Y,

tural dielectric sheets'. ory shapesl and' method of manufacture which arek hereinafter described and more vparticularly defined in the accompanying claims. f K A `In the following'description of a preferredform of the invention, reference will be made to the accompanying drawing inLwhich:

',Fig. il 'presents acurve in which the impact strengths of dielectric panels made up as herein described, are plotted against the'proportions of :liber and resin inv their composition;Y I

Fig.`2 presents a curve showing how thetrans-V vfers'e strength or modulus of rupture is alfected by 'changes inthe proportions of liber and resin;

Fig. 3 presents a curve plotting changes in densitylwith changes in proportions of liber and resin;-

, Fig'. 4 presents a curve plotting changes in mod ulus of elasticity with changesv in proportions of flberand resin;

Fimv 5 presents varied considerably, andthat "they may incorpo;

rate' plasticizing and'm'odifying agents and may 'be preparedA by condensing 'and polymerizing re` actions involvingv r hydroxy-aromatic homologues of phenolY and "compounds including formaldehyde having a reactive"'methylene' radical; The preferred mixture incorporates approximately 62.5% by weightof crocidolitebers andv 37.5% of phenoi-formaldehyde ltype* 'resinr The fibers and resin are thoroughlymixedin adry state to ef fectsubstantially uniform dispersion of the resin particles over the exposed surfacesof theffluiied fiberz: This mixing operationgrnay be carried out in a. rotary paddle type mixer-or in a hammer mill type of mixer. From the mixer the thoroughly dispersed, dry mixture of bers and heat hardenable resin may be conveyed to a preforming or cold molding unit or directly to a hot press moldlg and Curing. .ntf ,A f Y Wet mixing is avoided because it has been found that whenV the resin binder is incorporated in the resin-fiber mixture as` a liquid, it is diiiicult to produce amixtu're of uniform composition, sincev the liquid resins arek comparatively viscous, and since thev volume of resin is small in comparison.

' with thevvolume of highly lluffed asbestos liners.

a curve v plotting changes'Y 55 Moreover, whena solvent is employedlto increase theavolume and .to decrease the viscosity of the resin binder, an additional process step is required to remove this solvent, and this solvent removal operation, and the problem of replacing or recovering the solvent, adds substantially to the expense of the process.

face areas of 15 to 30 sq. ft. and varying in thickv ness from 1/4.-2 inches. 'The dry molding mii;- ture of opened and ilufi'ed crocidclite bers and resin particles which is required for producing a panel of the indicated size is comparatively bulky.' The preferred method of operation therefore contemplates a step of the dry mixture oi' asbestos i'fibersand resin binder l the sheet y of the resin'A under 'neat under high pressure in a cold mold, as a pirelli/nie nary to completing the consolidation of and cure hardening pressure. ing a panel of l inch thickness, for example, the dry resin-fiber mixture is preferably subjected in a batch mold at normal temperature to pressures of at least i500 lbs. per sq. inch and preferably to pressures Vof 3000 vor more pounds per square inch. In this preforming operation, the bulky mixture of bers and resin is compressed and shaped to a rigid handleable preform mat or sheet which may have substantially the ultimate face dimensions of the panel and which may have a thickness of ysay 21/4 to 3 inches. Since the preforming operationY is applied directly to the lurfednber-resin mixture, there is developed a certain heterogeneous or random lay of the fibers in the c-o-ld preform shape. By reason of this random lay ofthe fibers' and the preforming operation on the full charge of fiber-resin mixture, the final product has a monolithic structure which is more rmiform in Strength than a laminated product of the same generalcomposition. i

Preform snape's of the indicated range of thickness may be reduced to approximately 1 inch thickness during a final curing operation in a hot press mold under a pressure of at least approximately 2000 lbs. per square inch pressure, and under a curing temperature of approximately 30G-350 F. The final shaping and curing op?- eration may be carried out/or at least initiated, in a hot press mold having' confining side walls which are dmensioned to the final dimensions of the panel which it is desired to produce. Because of the large size and thickness of thepanels Whichare manufacturedin accordance with the present invention, a prolonged curing period of at least 30 minutes to 1 hour is required tadevelop complete cure-of the resin throughout a sheet of say 1 inch thickness. After forming and initiating cure of the sheet. by molding under pressure and elevated temperature, the cure of the `resin can be completed by oven heat treatment.

The curing temperature and pressure must be such as to soften the resin Yand to insure complete filling o-f the mold by the resin-liber miX- ture. The curing temperature and pressure must also be such as to consolidate and densifythe mold charge and to effect polymerization reaction hardening of the resin binder.A Some variations in both temperature and pressure may be necessary in employing other types of bonding resin and in molding sheets or shapes of difierent dimensions.

The cured shape (Fig. 6) has a smooth hard surface and a dense monolithic structure in which the crocidolite asbestos fibers l are disposed in heterogeneous or random intermeshed compressing andpreforming n rlhis low pressure curing period is then followed ln a preforming operation for produc` 1 reduced pressure heating er panel is held at curing lay and uniformly coated with and bonded by the phenolic resin binder l-2.

The curing cycle preferably includes a short breathing period under reduced pressure. For example the full cycle may include initial curing between hot platens at a temperature of approximately 300 F, andunder 2,000 pounds per square inch pressure fora period of say 5 minutes. This period of initial cure is followed by a period of during which the sheet temperature under a reduced pressure not exceeding about 500 pounds per square inch for a period of say 3 minutes.

by prolonged final curing treatment at the indicated molding temperature and under full mold pressure of about 2,000 pounds per square inch. The final curing period may last as long as 45 to minutes, depending on the thickness of the panel.

Appended hereto is a tabley illustrating the effeet on the properties of cured sheets or panels of comparable size when employing crocidolite asbestos fibers, in place of chrysotile asbestos fibers, as the reinforcing element. By replacing chrysotile fiber in the same formula with crocidolite fiber, the impact strength of the resulting panel is increased from 0.4 ft. lbs/cu. inch to 0.9 ft. lbs/cu. inch. The dielectric strength of a` sheet reinforced with crocidolite fiber is volts per mil, as compared to only 37 volts per mil for a sheet reinforced with chrysotile fibers. The sheets incorporating chrysotile fib er as the reinforcing material exhibit high electrical leakage near the breakdown value when subjected to electrical stress. Whereas the sheets made with crocidolite fiber display relatively small electrical leakage under the same conditions.

Fibro-cementitious sheets and panels of suitable dielectric and structural strength for severe naval surface 'can best be manufactured from compositions of the type 'herein described in which crocidolite asbestos fiber is the principal constituent. For maximumrstrength, it is essential that the crocidolitel asbestos be thoroughly willowed and opened, andthat'the average length of theiibers shall be not substantially less than about 1/4 inch. A suitable grade of crocidolite asbestos liber4 has been foundto classify as to length: 23.8% of 0.1jnch length or less, 25.1% of 0.1-0.2 inch, 24.6% of 0.2-0.3 inch, 16.7% vof 0.3-0.4 inch, and 9.8% of 0.4-1.0 inch. To produce a structural dielectric panel 'which exhibits optimum impact and flexuralV strength, dielectric stability, and optimum low moisture absorption it has been found that the proportions of fiber and resin in the Vcomposition are critical. Referring to the charts presented in Figs. 1 5 of. the drawing, curve A shows that panels or sheets in which the fiber proportions lie within the range (S0-70% ber and :iO-40% resin exhibit maximum impact strength. Curve B shows that the maximum modulus of rupture or flexural strength is also exhibited by panels having a composition of about fibers and 35% resin. Curves C and. D Vshow that the maximum density and the maxposition for crocidolite fiber -B stage phenolicA resin panels is approximately S21/2% fibers and approximately 371/2% resin. l

The physical properties of cured panels, as plotted in the curves of Figs. 1-5, were obtained by standard test procedures. Information for plotting the curves of Figs. 2 and 4 was obtained by applying a load to the center of a test panel strip having a 9 inch span between supports, at the rate of V2,000 lbs. per minute, until failure occurs. The center deiections. were determined at uniform Vincrements of load. The impact resistance, as plotted in the curve of Fig. 1, was determined for test panels by measuring the force of a bloW which will just causefailure in one blow, such blow. being delivered to the center of a 9 inch span by a Weight rounded at its striking end to a 1 inch radius. The impact resistance is reported as unit energy (single blow) per unit volume of sample. Water absorption was determinedV by immersing test specimens in water for a period of 48 hours at room temperature. Brinell hardness tests were made on the hard surface of a cured panel by measuring indentations eiected by application of a 1500 kilogram load through a 10 mm. Ysteel ballV over a period of 10 seconds.

After curing, the panel sheets normally exhibit a Brinell hardness number of approximately 100,

' an impact strength of approximately 0.9-1 ft. lb.

. tion after 24-48 hours per cu. inch, a flexural strength of approximately 24,000 lbs. per sq. inch, a density of approximately 130 lbs. per cu. ft. and a maximum water absorpimmersion of about 0.2- 0.3%. Such sheets or panels have a modulus of elasticity in the neighborhood Aoi 3 million lbs.

. per sq. inch, and have a dielectric strength of at least vl per mil thickness.

Since many variations may be made from the illustrative details give Without departing from the scope of the invention, it is intended that the invention should be limited only by the terms of the claims interpreted as broadly as consistent with novelty over the prior art.

What I claim is: n

1. A hard and tough dielectric panel having an impact strength of not less than 0.9-1.0 ft. lb. per cubic inch, a dielectric strength of at least 55 v. per mil thickness, and a exural strength exceeding 22,500 lbs. per sq. inch, and comprising '70% by weight of long crocidolite asbestos fibers, said bers being coated and bonded together with 3040% of heat hardened phenol-aldehyde resin.

2. A hard and dense fibro-cementitious panel of monolithic structure, comprising approximately by weight of crocidolite asbestos bers averaging 1/4 inch in length and arranged in heterogeneous lay, and approximately 35% heat hardened phenol-aldehyde resin binder, said shape having a maximum water absorption of not to exceed 0.4%, an impact strength of at least 0.9 ft. lbs. per cubic exceeding 22,500 lbs. sq. inch.

GEORGE SMOLAK.

inch, and a flexural strength 

